This invention relates to voltage control systems of the tap changing type, and more particularly to improved gating for suppression of arcing and circulating current as taps are changed.
A step voltage regulator is an autotransformer provided with load ratio control equipment for regulating the voltage on the feeder or bus to which it is connected. A typical step voltage regulator may have a one hundred percent exciting winding in shunt with the line on the source side and normally maintains the voltage on the load side within a desired voltage bandwidth by a 10% tapped buck/boost winding in series with the line. The series winding has taps connected to the stationary contacts of a tap changer dial switch having a pair of rotatable tap selectors driven by a reversible motor into sequential engagement with the taps and usually provides the ability to change the effective turns ratio from input to output plus or minus 10% in 32 steps of 5/8% voltage increments. In early designs, the rotatable selectors are connected through collector rings to the opposite sides of a bridging center-tapped autotransformer reactor, termed a preventive autotransformer, permitting transition from one tap position to another without interrupting the load current. The high impedence of the preventive autotransformer limits circulating current when the tap selectors bridge adjacent taps to a safe value and reduces burning and erosion of the tap changer contacts. It also provides a voltage midway between that of the physical tap to thereby provide twice the number of voltage steps. However, such a preventive autotransformer has continuous energy losses in operation and is bulky and expensive to construct.
The tap change regulator shown in U.S. Pat. No. 4,130,789 eliminates such preventive autotransformer and also prevents arcing at the tap changer selector contacts by providing a half-tap voltage auxiliary winding in an auxiliary switch which permits a selector contact to step arclessly to an open-circuited new tap, and then connect the selector contact in series with the auxiliary winding in a current-limiting inductor and the load at reduced voltage to effect a tap change without interruption of the load circuit. U.S. Pat. No. 4,201,938 discloses a regulator that prevents arcing, without utilizing a preventive autotransformer, by a shunt static switch circuit electronically ensuring that the main load current is interrupted at current zero. However, there is still some arcing at the auxiliary choke switching contacts, and there are still sizable circulating current losses in the regulator during tap changes.
U.S. Pat. No. 4,301,489 and U.S. Pat. No. 4,363,060, which are hereby incorporated by reference, disclose two methods for preventing arcing at the auxiliary switching contact by replacing the auxiliary current limiting choke with an auxiliary static switch circuit which by a combination of electronic and electro-mechanical control circuits ensure that the load current and the circulating auxiliary current are both interrupted at current zero. The regulators disclosed in these two patents provide good arcless regulation; however, there are still sizable power losses in the circulating current during tap changes and the electro-mechanical control signal switches must be installed and precisely aligned for proper timing with respect to the rest of the motor driven switches. Further, electro-mechanical switches are subject to wear and mechanical failures. Such would lead to mis-timing of the control signals and the result would be arcing and accelerated wear of the load and auxiliary switch contacts.
The tap change regulators shown in U.S. Pat. No. 4,301,489 and U.S. Pat. No. 4,363,060 use electro-mechanical switches for main current switching control, partly because of the stringent requirements for a totally electronic control system. An electro-mechanical control switch only senses for the main contactor position to determine if the main current switch control is required and thus is independent of the main current flowing to the load. An electronic control circuit instead of sensing the position of the main contactor, senses for zero rms main current as an indication that main current switching control is required, and thus must operate independently of main current whenever it is in the operating range between 16 amperes to 2000 amperes rms.
It is an object of this invention to provide an improved tap changer voltage regulator which has control circuits that are completely electrical in design to prevent the expense of installation and precise alignment of electro-mechanical switches during assembly and maintenance.
It is a further object of this invention to provide a more reliable arcless tap changer regulator by eliminating electro-mechanical control components for gating the static switch shunting the main switch and the static switch shunting the auxiliary circuit.
It is a further object of this invention to provide a main current static switch gating circuit that is independent of the main current over the entire operating current range.
Another object of this invention is to reduce the circulating current and the power losses associated with the circulating current during tap changing to less than one-half cycle of the alternating current fundamental frequency in duration.